The present invention relates to an electrode plate for a secondary battery with nonaqueous electrolyte such as represented by a lithium ion secondary battery and also relates to a method of manufacturing the same.
In recent years compact in size and light in weight, electronic equipment, communications equipment and the like, have been rapidly developed, and it has been required that the secondary batteries used for the driving electric power sources of this equipment also be reduced in size and in weight. According to, there has been proposed a secondary battery with nonaqueous electrolyte such as that represented by a lithium ion secondary battery having high energy density and high voltage.
Furthermore, concerning electrode plates which severely affect the performance of the secondary battery, it has been also proposed to make large an area of a thin layer so as to elongate charge-discharge cycle life and to make the energy density high.
In a prior art publication such as Japanese Patent Laid-open Publication No. SHO 63-10456 or Japanese Patent Laid-open Publication No. HEI 3-285262, there is disclosed a positive electrode plate which is obtained in a manner such that a paste-form active material coating solution is prepared by dispersing or dissolving a conductive agent, binder powder and a positive electrode active material such as metallic oxide, sulfide, halide or the like, in a suitable wetting agent, and the coating solution is coated on a collecting element (or collector) as a substrate formed of a metal foil to thereby form a coated layer (battery active material layer). In such a process, fluorine series resin such as polyvinylidene fluoride, silicone-acrylic copolymer or styrene-butadiene copolymer is used as a binder.
With the coating-type electrode plate of the character described above, it is required for the binder used at a time of preparing the coating solution containing the active material to be electro-chemically stable to the nonaqueous electrolyte, not to be dissolved in the electrolyte and to be dissolved in a certain solvent. It is also required that the coating solution can be coated in a thin layer form on the substrate formed of a metal foil. Furthermore, it is further required for the active material layer (coated layer) formed through the coating and drying processes to have a flexibility so as not to be peeled off, removed or cracked during assembly of the batteries and also required to have an excellent adhesion to the collecting element formed of a metal foil.
The coated layer constituting the electrode plate generally has a layer thickness of about 50 to 200 .mu.m per one surface thereof. The capacity of a battery per unit area can be increased by increasing this layer thickness of the coated layer.
However, when the thickness of the coated layer is increased, a convection flow is caused in the coated layer during the drying process, which results in uneven distribution of the binder amount in the coated layer after the drying process in such a fashion that the binder amount is decreased at a boundary portion of the coated layer contacting the collecting element (called contacting surface or contacting surface side hereinafter) and, on the contrary, the binder amount is increased at a boundary portion of the coated layer opposite to the contacting surface side. The latter boundary portion is exposed in air or exposed to an electrolyte when disposed in a battery (called exposed (or opposite) surface or exposed surface side hereinafter). In particular, in the case of the coated layer thickness more than 100 .mu.m, such uneven distribution of the binder amount becomes more remarkable, which results in the damage of the adhesion performance of the coated layer with respect to the collecting element. At the same time, the coated layer becomes easily peelable from the collecting element because of the lowering of the adhesion property, and accordingly, the flexibility of the electrode plate will be reduced. Thus, it becomes difficult to carry out a bending working of the electrode plate. Such uneven distribution of the binder at the boundary portion of the exposed surface side of the coated layer appears remarkably as the drying speed increases, and accordingly, in order to prevent the adhesion performance between the coated layer and the collecting element from lowering and the flexibility of the coated layer from damaging, it is necessary to slow the drying speed, which however results in the lowering of the productivity.
In another viewpoint, in a case where the weight ratio of the binder/active material is made larger than 0.25, it is possible to prevent the lowering of the adhesion performance at the time of the rapid drying of the thickened coated layer. However, the increasing of the binder amount leads to extreme lowering of the battery performance, thus this countermeasure is not practical.
As described above, in the prior art, it was difficult to produce an electrode plate with a coated layer being relatively thick, containing a small amount of a binder and, moreover, having an excellent adhesion performance to a collecting element.